Study of the substitution pattern of hydroxyethylcellulose and its relationship to enzymic degradation

Abstract

AbstractThe concentration of completely unsubstituted anhydroglucose (AHG) units in hydroxyethylcellulose (HEC) at varying average number of moles of substituent (MS) is in accord with a reaction rate ratio of 3:1:10:10 for the etherification of the C2, C3, C6, and side chain hydroxyls. Assuming random substitution along the cellulose chain at these relative reaction rates, the degree of substitution (DS), average poly(ethylene oxide) side‐chain length, and probably frequency of isolated and multiple adjacent unsubstituted AHG units have been projected as functions of MS. Such estimates of DS and side‐chain length compare favorably with actual measurements made on a variety of samples by other investigators below a substitution level of 2.0 MS. Reducing end, intrinsic viscosity, and ultracentrifuge data on enzymically hydrolyzed HEC's varying from 1.9 to 3.7 in MS indicate resistance to chain scission is a function of substitution, best expressed in terms of a proposed substitution index, the percentage of substituted AHG units. The rate of chain scission under carefully controlled conditions is initially rapid, tapering to a very slow, essentially steady level. On relating the frequency of chain breaks to the predicted substitution pattern of these samples, this behavior is interpreted to mean that scission occurs relatively rapidly at multiple contiguous unsubstituted sequences, but much more slowly adjacent to isolated unsubstituted AHG units. A possible mechanism of enzyme attack, correlating molecular chain breaks with glucose released, has been developed.